Determining the hiding power of paints and pigments



Sept. 18 1923.

A. H. PFUND DETERMINING THE HIDING POWER OF PAINTS AND PIGMENTS Filed Nov. 2l,

fio

Patented Sept.. ll, i923..

niet star AUGUST HERMAN PJEUND, 0F BALTMORE, MARYLAND.

DETEBMINL'NG THE HIDNG POWER iPNTS AND PIGMENTS.

Application led November 21, 1919. Serial No. 339,71.

To all whom t may concern Be it known, that ll, AUGUST HERMAN lFUND, a citizen of the United States, residing at Baltimore, in the county of Baltimore City, State of Maryland, have invented` certain new and useful Improvements in Determining the Hiding Power of Paints and Pigments; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates to improvements in determining the hiding power of paints and pigments, and has for its objects the provision of an improved method of obtaining a measure of the hiding power of a paint or pigment. as well as the provision of appropriate apparatus for carrying out this method.

The hiding power of a paint may be defined as that property of a paint which enn ables it to obliterate beyond recognition any background upon which itl may be spread. In order to compare the hiding power of two white pigments, the almost universal custom has heretofore been to rub down equal masses of the two'samples with the saine amount of linseed oil and ultra-marine blue. The result-ant astes are spread side by side and the samp e which has the paler tint is supposed to contain the pigment of greater hiding power. The idea is that the particles of white pigment hide' the dark particles and hence give a light. tint to the paste. As the result of investigations conducted in accordance with the present invention, it appears that the criterion, upon which the aforementioned tests have heretofore been based, is incorrect,

@ne of the objects of the'invention is to provide a method which will yield numerical values of the true hiding power of white paints and pigments. The basic idea underlying the invention may be expressed thus: Granting that an infinitely thick layer of a paint will hide a given background completely, it is sought to find the thinnest layer which will hide the back-ground as eRectively as does the infinitely thick layer. Obviousl the thinner the layer of paint requiredy the paint. For white paints, the severest test met with in practice is a white wall with black lettering. In such a case, it is desired to obliterate the lettering by covering the the greater is the hiding power of' entire surface with successive coats of paints. These conditions are simulated by the method and apparatus of the present invention.

In its general aspect, the method of the invention involves spreadingupon an appropriate back-ground two juxtaposed lay ers or portions of the paint or pigment, the thickness of one of these layers being greater than required to completely hide the background. By progressively varying the thick ness of the other or second layer of the paint or pigment until the hiding power of the second layer is substantially equivalent to that of the first layer, there is obtained a measure of the hiding power of the paint or pigment in terms of the thickness of this second layer of the paint or pigment when its hiding power is substantially equivalent to that of the first layer.

rihe novel features of the invention which l believe to be patentably characteristic thereof are set forth in the appended claims. These features, together with the construction and mode of operation of apparatus embodying the same, will be better understood from the following description taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a diagrammatic elevation of a sim le device for carrying out the method of t e invention;

Fig. 2 is a plan view of the device of Fig. 1;

Figs. 3 and 4 are elevation and plan views respectively of a forni of apparatus i have found well adapted for the practice of the invention.`

Fi 5 is a sectional elevation on the section line 5 5 of Fig. Li; and

Fig, 6 is a diagrammatic view of a screen which if have found useful in conjunction with the apparatus illustrated in the preceding figures.

Referring first to Fig. 1 of .the drawing, there is diagrammatically represented a plate 10 of glass whose upper surface is optically flat. The lower surface of the glass plate 10 is coated with black baking enamel 11, which yields the desired black back-ground for working with white paintY and pigments. A transverse groove 12 is cut in the upper surface of the glass plate 10 and an appropriate scale 13 is etched on the upper surface of the glass plate, as shown in Fig. 2 of the drawing. i have used, with advantage, a plate of glass 1,4 x x 0.6 centimeters having a transverse groove l 2 mm. deep and 1 centimeter wide.

Resting upon the plate 10 is a glass plate 14, whose lower surface is likewise optically fiat. A strip of thin steel 15 is attached to one end of the plate 14, so that a wedgeshaped layer of paint may be formed between the adjacent surfaces of the glass plates 10 and 14. I have successfully used for the plate 14 a piece of glass 7 X,3.5 X 0.6 cm. having a strip of steel 0.45 mm. thick attached to one end thereof.

The wedge-shaped layer of paint between the glass plates 10 and 14 terminates abruptly at the infinitely thick layer or portion A, contained in the groove 12, and, so long as the vhiding of the juxtaposed layer or portion B of the wedge-shaped layer of paint is not complete, the line of demarcation between these two layers will be visible.

By sliding the plate 14 to the left, as viewed in Figs. 1 and 2 of the drawing, it finally becomes impossible to see the edge or line of demarcation between the juxtaposed layers A and B of the paint. From a knowledge of the angle of the wedge and the reading on the scale 13, it is possible to' calculate the thickness of this critical layer lying immediately above the edge C, this edge C being the line of division between the infinitely thick layer of paint A and the juxtaposed wedge-shaped layer of paint B.

Now, in advancing the plate 14 until the line of demarcation can no longer be seen, we have overdone it, so to speak. To correct this, we must reverse the motion of the plate 14 until the edge C can just be distinguished over its entire length. The mean value ot the readings, corresponding respectively, to

the disappearance and the'appearance of the edge C yields the desired result. Since the fading away and reappearance of the edge C is so gradual, due to the fact that the least perceptible increment of intensity which the human eye can detect is 1 to 2% (Fedners law), it is clear that no high degree of precision is attainable b this method. By taking ten pairs of rea ings, it has been found that the average deviation from the mean is about 3 to 4%.

In Figs. 3, 4 and 5 of the accompanying drawing, I' have illustrated a practical forni of the device which I had used' with success. In this modification of the invention, the base plate 10 of glass or other appropriate transparent material is mounted on a block 16 which may advantageously beef wood. 'Ihe plate 10 is surrounded by a rectangular frame 17, which prevents the paint or pigment from slopping over. 'The base plate 10' hasl a recess 12', which is adapted to contain the infinitely thick layer of the paint or pigment. The movable plate 14, of glass or other appropriate transparent material,

Ytend to limit the invention to these slides within the frame 17, transversely of the recess 12. The plate 14 has a thin strip of metal 15 attached to one end thereof so as to provide the desired wedge-shaped s ace between the adjacent surfaces or faces o the plates 10 and 14. An appropriate background 11 is formed on the lower face of the base plate 10.

The device shown in Figs. 3, 4 and 5 is of the duplex type, so that two lots of paint or pigment may be tested at the same time and thus their respective hiding powers conveniently compared. To this end, a transverse strip 18 is fitted in appropriate grooves in the plates 10 and 14 and is arranged substantially midway of the width of these plates. Two scales 13 are provided for the two -testing sides of this duplex device.

A suitable viewing device has beenl found of assistancesince the reflection ofthe observers vface in the upper plate14 is annoying.- A suitable arrangement' for this purpose is diagrammatically illustrated in Fig. 6 of the drawing. As indicated in Fig. 6, strong daylight illuminates the apparatus from the observers right (or left), the light rays being represented in this figure by the dotted lines a. The viewing device comprises an appropriate base 19 upon which is mounted a housing 20 having its inner walls blackened. The housing 20 is provided with an appropriate opening 21 through which the superposed plates 10 and 14 may be seen. At its back, the viewing device is provided with a vertical blackened)` shield 22. I have successfully employed for the housing 20 a box-like chamber of about 30 cm. in height.

For convenience, I call the device or instruinent for determining or measuring the hiding power of a paint or pigment, as hereinbefore described, a cryptometer from the two Greek words vfor conceal and measure The instrument may, if desired, be developed to yield results of high vrecision. I will` herein refer to two methods y which this result maybe accom lished, 'although ity will be understood that do not thereby inarticular methods of securin high precision. If we call the hiding comp ete when the brightness of the variable layer is 99% of that of the infinitely thick layer, then, by means of a photo-electric cell, which measures the light from the two layers in contact at the edge C, it is possible to locate the desired point very accuratel Again, by making, preliminarily, a stu y of the variation of brightness of the paint or pigment with increasing thickness, a formula can be established connecting these 'two quantities. Then, by finding a position of the movable plate 14 where the brightness is, say, onehalf that of the infinitely thick layer, it is possible to calculate the thickness yielding incanta 99% of the brightness of the infinitely thick layer. F or most purposes, high precision is not necessary, and for this reason, the use ot the cryptometer, in the form and manner herein illustrated and described, is not only suciently accurate but is extremely simple in its construction and operation.

A sharp distinction must be made between the hidin power of a pigment and that of a paint.- ot only are these quantities expressed in diderent units lbut they are notv necessarily related in the sense that a pigment .of great hiding power necessarily roduces a paint of correspondingly reat iding power. Taking up irst the hiding power of pigments, let us consider an intimate mixture of grs. of a white pigment and y grs. of colorless (or very pale) linseed oil. This mixture is tested in the cryptometer and the critical thickness producing complete hiding is found. Let

tzthickness of critical layer (in cms).

bznumbers of grs. of pigment in a disc of 1 cm2 base and thickness t.

Then, if b grs. pigment hide 1 sq. cm., we find the number of sq. cm. A covered and hidden by one gram of pigment trom the relation:

b:1::1:Aor A `,ner the layer, that is, the smaller b is, we

may dene the hiding power of a pigment as the reciprocal of the number of grs. of pigment, mixed with colorless linseed oil to painting consistency, which are necessary to hide a black, non-absorb-end area 1 cm2. This is numerically equal to the number ot sq. cm. covered and hidden by 1 gr. of pigment. Hiding powers of pigments will, therefore, be expressed in terms of cm2 per gr. llnvestigations have shown that the hiding power of the pigment is edected by the relative amount of oil present :-the thinner 'the mixture, the greater the hiding power. This means that the hiding power is not only a function of the number of white particles, but also of their separation. lln order to'reduce all mixtures to a standard condition the term aintin consistenc ple. The paint is measured up in the cryptometer and, from a -howledge of the critical thickness, the number of square'feet per gallon is calculated at once. As a matter ot tact the cryptometer may, ot, course, be

tin)

made so that one may read the result directly od a separate scale etched on the lower plate.

The resultsk obtained tor characteristic pigments and paints are summarized in the following table:

Concerning the pigments themselves, the results obtained speak for themselves. The one point, however, to which attention must be called, is that involving a comparison of pigments 6 and 7. The only didierence between these two pigments is that 6 is white while 7 is pearl-grey. Tf these same pigments are compared by rubbing them down with oil and ultra-marine blue, the grey sample will show the darker tint, hence it will be considered as having a smaller hiding-power than the white sample. As a matter of fact, the hiding-power ot the grey sample is more than three times as great as that of the white. llt is therefore clear that, since the rub-down test yields results which are so violently at variance with those obtained with the cryptometer, the former method should be discarded.

The results on paints are interesting. While the hiding-powers ot zinc pigments are larger than those ot the white leads, the reverse is true ot the paints. (Compare samples 2 and 6). The reason is obvious when the oil-absorption of the pigments is considered. @ne pound ot zinc-oxide mined with one pound oic linseed-oil yields a mixture of proper painting consistency, while a similar mixture of white-lead in oil is entirely too runny for painting purposes. More lead must be added to give the correct body or thickening to the paint. Such a paint has the greater hiding-power but it has gained its superiority in consequence of the excess of lead pigment which has been added.

Tt is clear that it a surface of lighter tint i lltltl lllti) 'liti `relatively great thickness,

this plate be rested on the actualv surface to be painted, it is again possible to determine in advance just how many gallons are required to paint a given area.

From the foregoing discussion, it will be evident that the invention provides a method and an instrument which yield numerical values of the hiding-powers of paints and pigments. Both the method and the instrument are simple in operation, and can be utilized with convenience. The results obtained from the practice of the invention are, moreover, accurate and reliable, and enable accurate "and reliable predictions to be made of the behavior of variouspaints and i I claim y pigments in. actual painting practice.

1. The method of obtaining a measure of the hiding power of a paint or pigment which comprises spreading upon an appropriate background two juxtaposed layers of the paint or pigment, one of said layers being of greater thickness than required to completely hide the background 'and the other layer being of adjustable thickness, progressively varying the thickness of said adjustable layer of the paint or pigment until the hiding ower of this layer is substantially equiva ent to that of the other layer, and indicating the hiding power of the paint or pigment in terms of the thickness of said second layer when its hiding power is substantially equivalent to that of the other layer.

2. The method of obtaining a measure of the hiding power of a paint or pi ment which comprises forming a wedge-s aped layer of the pait or pigment in juxtaposition to a layer of the paint or pigment ot varying the thickness of vthe juxtaposed edge. of said wedge-shaped layer until the line of demarcation between said two juxtaposed layers is just perceptible, varying the thick- 'nessof the juriftaposed edge of said wedge'- shaped layeruntil the line of demarcation between said two juxtaposed layers just disappears, and obtaining a measure of the hiding power of the paint or pigment from the mean thickness of the juxtaposed edge Vof said wedge-shaped layer after each of `the aforementioned procedures.

3. The method of obtaining a'measure of the hiding power of a paint or pigment which comprises spreading above an appropriate background two juxtaposed layers of the paint or pigment, one la er being of greater thickness than require to completely hide the background and the other layer being of adjustable thickness, pro` gressivelj increasing and decreasing the thickness of said adjustable la er until the hiding power of the juxtapose edge of the variable layer is substantially equivalent to the hiding power of the other layer, and obta-inin a measure of the hiding power of the palnt or pigment from the mean thickness of the juxtaposed edge of the adjustable layer after increasing and decreasing the thickness of this layer until the hiding power of the juxtaposed edge of the variable layer is substantially equivalent to that of the other layer.

4.' A device of the character described, comprising a pair of relatively movable, superposed, transparent plates having a wedge-shaped space and a recess therebetween, and an appropriate background on the face of one of said plates.

5. A device of the character described, comprising a transparent base plate having an appropriate background on one of its faces and a recess in its other face, a second transparent plate operatively positioned on the recessed surface of said base plate and movable with respect thereto, and Ymeans providing a wedge-shaped space between the adjustable faces of said plates.

6.. device of the character described, comprising an appropriate background, means for maintaining a relatively deep layer of paint or pigment above the background, means for maintaining a layer of paint or pigment of adjustable thickness above the background in juxtaposition to said relatively deep layer, means for adjusting the thickness of said second layer of paint or pigment, and means for obtaining a measure of the thickness of said second layer of paint or pigment in any of its aforementioned adjustments.

Y., A device of the character described, comprising a glass base plate having a. black, opaque background on one of its faces and a transverse recess on its other face, a second glass plate operatively positioned on the recessed surface of said base plate and movable transversely of the recess, and means providing a wedge-shaped space between the adjacent faces of said plates so that the thickness of said space in the vicinity of the` recess may be varied by the movement of the second plate transversely of the recess.

In testimon whereof I aix m. signature.

AU UST HERMAN FUND. 

